1. Field of the Invention
The invention relates to an exhaust-gas cleaning system of an internal combustion engine, having a device for selective catalytic reduction. The invention also relates to a method for cleaning exhaust gases of internal combustion engine, in which an exhaust-gas stream is carried through a device for selective catalytic reduction.
2. Prior Art
To reduce the nitrogen oxide components in oxygen-rich exhaust gas, of the kind emitted especially by Diesel engines and by direct gasoline injection engines, it is known to introduce a reducing agent into an exhaust gas stream. NH3, for instance, is suitable as a reducing agent and can be introduced in the form of gas into the exhaust-gas stream. In this selective catalytic reduction (SCR), the ammonia, along with the nitrogen oxides contained in the exhaust gas, is selectively converted into molecular nitrogen and water. Because of its toxicity, however, pure ammonia gas is unsuitable for use in the motor vehicle. One known method provides for the use of aqueous urea solution as a reducing agent. In that case, the actual reducing agent, ammonia, is not released until after thermolysis and ensuing catalyzed hydrolysis of the urea.
The known SCR systems have inadequate activity at exhaust gas temperatures below about 250° C. Connecting an oxidizing catalytic converter to the upstream end assures a reduction in the proportion of hydrocarbons, which have a deactivating effect, on the one hand and an oxidation of NO into NO2 on the other, the overall result being a marked increase in the conversion of NOx at exhaust gas temperatures above about 200° C. Below about 180° C., because of the relatively long decomposition time of urea to NH3, the system offers only inadequate activity. Particularly in use in the passenger car, however, phases with such low exhaust gas temperatures are relatively frequent, as is clearly demonstrated by a mean catalytic converter temperature of less than 180° C. in the so-called MVEG test cycle.
To assure good distribution of the reducing agent in the SCR catalytic converter, a mixing distance of approximately 40 cm, optionally provided with a mixing device, may be provided. Such a mixing device for an exhaust-gas cleaning system is described in the earlier German Patent Application, Serial No. 101 31 803.0. In it, a mixing body disposed in the exhaust gas tube has a gas impact face as well as a stream impact face, so that exhaust gas flowing out of the engine can strike the gas impact face, and reducing agent which can be delivered transversely to the exhaust-gas stream can strike the stream impact face.